1. Field of the Invention
The invention relates to optoelectronic devices and in particular to a bonding pad structure for an optoelectronic device and fabricating method thereof.
2. Description of the Related Art
Digital image devices are widely used in, for example, digital cameras, digital video recorders, cellular phones with image capture function, and monitors. A digital imaging sensor typically includes an optoelectronic device chip, such as a charge-coupled device (CCD) image sensor chip and CMOS image sensor chip.
Such an image sensor chip typically comprises metal structures used as electrodes for picture elements and as bonding pads for electrical connection with external circuits. The metal structure is formed by known damascene process and metallization. A barrier layer is typically formed prior to metal filling. That is, the barrier layer is formed on the bottom surfaces and sidewalls of the metal structures. However, the atoms of the barrier layer may diffuse into the bottom surfaces and sidewalls of the metal structures during performing thermal process in device fabrication.
In a back-illuminated image sensor, the image sensor chip comprises a device substrate having micro-electronic devices formed thereon, and reversely mounted on a carrier substrate or a protective substrate. Since the device substrate is reversely placed, portions of the device substrate must be removed to expose the metal structures serving as bonding pads for subsequent wire bonding process. However, as the device substrate is reversely arranged on the carrier substrate and partially removed to expose the bonding pads, the bonding wire may contact the surface of the bonding pad with impurities diffusing from the barrier layer, resulting in poor adhesion between the bonding wires and the bonding pads. Poor adhesion between the bonding wire and the bonding pad may reduce package reliability and cause device failure.
Thus, there exists a need for a bonding pad structure for an optoelectronic device with enhanced bonding pad adhesion.